AnActiveX vulnerability used by hackers to exploit Microsoft's Internet Explorer browser is a prime candidate for another Conficker-scale attack, security experts said.

AnActiveX vulnerability used by hackers to exploit Microsoft's Internet Explorer browser is a prime candidate for another Conficker-scale attack, security experts said.

On 6 July, just hours after security companies reported that thousands of compromised sites were serving up exploits, Microsoft acknowledged the flaw in the ActiveX control that can be accessed using IE. The bug has been used by hackers since at least 9 June. Microsoft said it will issue a patch for the flaw on July 14.

The vulnerability "exposes the whole world and can be exploited through the firewall," said Roger Thompson, chief research officer at security software vendor AVG Technologies USA. "That's better than Conficker, which mostly did its damage once it got inside a network."

Conficker exploited a Windows flaw that Microsoft had thought dire enough to fix outside its usual update schedule in October 2008. The worm exploded into prominence in January, when a variant infected millions of machines that remained unpatched.

Microsoft confirmed the latest flaw shortly after security researchers at Danish firms CSIS Security Group AS and Secunia said that thousands of hacks of legitimate websites over the July 4 weekend had exploited the bug.

The hackers took advantage of the bug to reroute users to a malicious site, which in turn downloads and launches a multiexploit hacker tool kit.

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Two days after disclosing the bug, Microsoft admitted that members of IBM's X-Force threat research team had first reported the vulnerability to it sometime in 2008.

The X-Force researchers had uncovered the flaw in late 2007, and in December of that year reserved a number in the Common Vulnerabilities and Exposures list of publicly known information security vulnerabilities.

One of the researchers, Alex Wheeler, now manager of 3Com's TippingPoint DVLabs, declined to say exactly when the flaw was discovered, citing a non-disclosure agreement he had signed with his former employer.

A Microsoft spokesman didn't say why the flaw wasn't patched earlier.

"When we were alerted in 2008, we immediately started an investigation," the spokesman said in an e-mail to Computerworld. "As we wanted to be thorough, this took extra time to fully evaluate."